This invention relates generally to color printing and more particularly provides a method and apparatus to effect synthesis of color proofs from screened color separation positives employing electrophotographic methods.
Color proofs for the printing industry primarily have been made by silver halide photographic techniques using plural filter media for making color separated prints or color composite prints. The results obtained have been generally satisfactory but are capable of improvement. Electrostatic techniques have been suggested but generally involve single-use electrophotographic techniques and require complex, expensive equipment. In some instances, separate equipment may be required for different stages in the process and often are slow, expensive and provide low quality reproductions. Those processes known to the art for producing good results, often produce results of quality much higher than is desired or economically useful.
Generally speaking, the making of color copies by electrostatic techniques has in the past involved the steps of optically projecting color resolved images of the original onto a single charged electrophotographic member, one for each color resolved image, so as to form corresponding latent electrostatic images. These latent electrostatic images are developed to a desired (proper) color. The image is transferred to a secondary substrate, such as paper or the like. The color resolved images projected onto the electrophotographic member or members were obtained by projecting an image of the original onto the charged electrophotographic member or members through a plurality of color separation filters, one for each color to be reproduced, or by silver halide techniques.
Electrostatic systems which have been proposed for use in making color proofs, for the most part, have not included the capability for making changes in the color, size or hue of the final copy, if such changes are either desired or required except by very difficult or expensive means involving changes in the process.
In U.S. Pat. No. 4,025,339 which issued on May 24, 1977 to M. R. Kuehnle, there is described an electrophotographic film having qualities and properties which make it far superior to any other known electrophotographic film. The electrophotographic film disclosed in that patent features a photoconductive coating that is wholly inorganic, microcrystalline, electrically anisotropic in nature, which does not have reciprocity or intermittency effects, operates at low voltages, has very high sensitivity and can produce toned images of exceptionally high quality from either analog or digital information. The photoconductive coating may comprise a layer of RF sputtered cadmium sulfide having a thickness of the order of 3,000 to 10,000 Angstroms. Because of its exceptional properties, the electrophotographic film is particularly suited for use in making high resolution color copies of color originals by electrostatic techniques, and, in fact, has properties which enable it to be used for this purpose in a manner that hitherto has not been achievable with other types of electrophotographic members.
Because of the high speed of response of said coating (capable of being exposed in nanoseconds), the invention can be embodied in a relatively high-speed machine to provide color proofs much faster than heretofore possible.
Starting out with a color separated film original, one known process employs a master paper which carries thereupon an electrophotographic coating having photoconductive properties. The master paper is positioned on a platen employing register pins or the like, said platen being part of an electrostatic charging unit. An electrostatic charge is applied uniformly over the entire surface of the master paper. The separation film is placed in proper registry on the charged electrophotographic member and exposed to light, the non-imaged area discharged and the charge being retained in the imaged areas to define an electrostatic latent image. The master sheet is developed with toner of the proper color. After rinsing following development, the toned master sheet is removed from the charging and exposing apparatus and introduced into a separate multiroller transfer unit for transfer of the toned image to a substrate. The transfer normally is effected by first transferring the image to a cylindrical surface electrostatically and thereafter, from the cylindrical surface to the substrate. A separate drying unit is provided for evenly and thoroughly drying the proof sheet after completing each color cycle.
One problem encountered with the last described type of system is that the electrophotographic master can only be used once. Additionally, physically separated units were required for the performance of each stage of said proofing process.